The Nenets of northern Siberia: 'What made us survive all these hundreds of thousands of years is our spirituality; the link to our land.' Photograph: Sebastião Salgado/Amazonas/nbpictures

Eight years ago, at the age of 60, Sebastião Salgado set out on his most ambitious project. The photographer, best known for his painterly portraits of migrant communities and manual workers, decided to document the world's pristine territories – areas untainted by the brutal grind of industry, exploitation and modern life.

Through his work, Salgado had seen so much horror – in the Democratic Republic of the Congo, he witnessed 10,000 people die in one day in the refugee camps of Goma. The playwright Arthur Miller once described Salgado's pictures as an act of deep devotion, but the photographer returned from Exodus, a project about people fleeing genocide, with his faith in tatters. "I was injured in my heart and my spirit," he says. "I came away from this with incredible despair." So he embarked on Genesis, hoping it would be restorative, a celebration of the world's natural beauty.

Thirty trips later, Salgado has finished Genesis. He has lived among the giant tortoises and marine iguanas of the Galápagos Islands, the mountain gorillas of Rwanda, the naked, pacifist Indians of the southern Amazon, the brown bears of the Kamchatka peninsula. Now he is finishing the book, designed by his wife, Lelia, and already he sounds bereft. "I am a bureaucrat, putting together the book, writing captions…"

I ask what Genesis has taught him, and he doesn't know where to start. "Many things. The first truth is, I have been told a lie throughout my life. We were always told we were the only rational species. But each species has its own rationality, the same as the trees, the plants: they all have incredible behaviour, incredible rationality, incredible intelligence."

This visit, to the nomadic Nenets of northern Siberia, was one of his final trips. There are approximately 42,000 Nenets and their lives are defined by reindeer, which are the source of their food, clothing and transport. Winter is spent in their communities, near the Kanin and Taymyr peninsulas, around the Ob and Yenisey rivers. In summer, when the weather gets too hot for the reindeer, the Nenets head north into the Arctic Circle, where the reindeer dig under the tundra for grasses and hardy vegetation.

Salgado says the animals are fantastically smart. "They can go down 70 or 80cm beneath the snow, and they know exactly where the food is. When we crossed the Ob, the biggest river in Siberia, the sky is white, the ice is white, everything is white. The Nenets were completely lost, but the reindeer were not. They drove everyone to the other side. Incredible." Normally, the reindeer amble along at 15km a day at most, but when crossing the Ob they manage 50km in one day, because they know there is no food under the ice and if they slow down they will starve.

Salgado travelled with a group of 18 Nenets. The word he comes back to again and again is "essential". Every five days or so, the hunters would kill a reindeer. If they cut the animal's throat, they lose out on essential protein, so they strangle it, drink the warm blood and eat the heart, kidney and liver raw. They then skin the animal – another essential resource. The Nenets' coats comprise four layers of reindeer skin to shield against temperatures of -30C; their boots are made from silver fox fur. They store the reindeer meat in the snow and eat it when required.

The only thing approximating luxury is vodka, but the Nenets might well say that is an essential, too. They buy fish and vodka using reindeer meat and horn as currency. (Powdered reindeer horn is regarded by the Chinese as an aphrodisiac.)

How did Salgado cope with the climate? "The cold was a problem," he admits. "The clothes I brought with me did not work. They were guaranteed for -50C, but when I got there it was -35C, and after two or three hours you start to feel cold." The Nenets made him a coat and from then on he was comfortable outside for 10- to 12-hour periods.

In Salgado's 40 days in Siberia, he did not wash. "The Nenets wash themselves once a year, in August. After that, they are good for another year." Do they smell? "No! If everybody smells like this, there is no smell."

It sounds as if their entire life is predicated on survival and little more, I say. Oh no, Salgado says, that's not true. After a day's travelling, they would build temporary wooden homes, or tchoums, cook their only hot meal of the day on a wood-fired stove, drink vodka and have long conversations. "There is so much love in their lives: wife to husband, husband to wife, for their children. Everything around them makes their life very rich, and they tell each other such nice stories."

Salgado has come away from Genesis more convinced than ever of our need to re-establish our relationship with the natural world. "The big problem today is cities. We are animals, born from the land with the other species. Since we've been living in cities, we've become more and more stupid, not smarter. What made us survive all these hundreds of thousands of years is our spirituality; the link to our land. Today, we are isolated, and we must do something about this."

Should we live like the Nenets? Salgado laughs. "No. We cannot go back to the way they live. They've come over much more to our side." He talks about how many of the children now go to school, and says the parents feel ambivalent – they want their children to have an education, but they miss them while on their travels. Some Nenets stay behind to work in the small towns. "I looked at the men who weren't going on the trip and something was breaking inside them. They were like, 'Look at those guys going to paradise' and they no longer had this life."

Nor, indeed, does Salgado. What is it like to be home from his travels? "I feel empty. I want to go back." If he could return to one place he visited for Genesis, where would it be? "Ethiopia," he says instantly. "I did an 850km walk over 55 days, from the holy city of Lalibela to Gonda. It was the most fabulous trip I did in my whole life."

Eight years ago, Salgado said Genesis would probably be his last epic project. Is that still the case? "Let's see." He pauses. "Probably not." Another pause. The passion is back in his voice: "I don't know what the new one will be, but I am sure it will not be the last." Simon Hattenstone

Sebastião Salgado is embarking on the last of his great photographic projects, which will appear regularly in Weekend over the next eight years. He is seeking out places that are still as pristine as they were in primeval times, places that provide hope